Improve flex layout

[liZe]

This meta-issue is sponsored by 💖 the NLnet Foundation 💖.

Let’s rewrite the whole Flex layout, and fix (hopefully) all these old and dirty issues!

All the work is done in the flex branch until it is merged before releasing v65.

[liZe]

It’s time for a little retrospective with numbers!

• We closed 18 issues, including a lot of bugs but also nice features like gap.
• Closing these issues helped to fix unrelated bugs, for example for inline tables, preferred sizes, floats, formatting structure… We even fixed a bug in one of our samples!
• We added 123 tests to our test suite.
• 346 flex layout tests from the W3C test suite were passing in version 64, they are now 470 (out of 810). Many of the issues left are not related to flex layout but to other unsupported features (XHTML syntaxes, right-to-left and vertical text mainly).
• We pushed 47 commits in 3 weeks.
• We rewrote about 35% of the flex layout code, and the flex.py file is now 180 lines shorter (we removed a lot of comments copied from the specification).
• The overall diff stat is +517-684 for code in 11 files, and +1324-343 for tests in 3 files. We basically removed broken code and transformed it into passing tests! 😄

Thanks a lot to 😍 the NLnet Foundation 😍 and to 😻 all the bug reporters, testers and contributors 😻.